A so-called Sight Well type sludge collection system as seen in prior art and references cited in U.S. Pat. No. 4,193,877 includes an open-top rotatable box generally cylindrical or rectangular in shape which is partially submerged in the tank liquid/solids and surrounds an upper portion of an influent column. The sight well has suction pipes depending therefrom which connect to radial rake arms rotatable to sweep around the tank bottom. A controlled flow of settled sludge is transported by a hydraulic differential force into the open well from whence the sludge moves through inlet ports in the influent column to fall down a sludge pipe, concentrically mounted inside the influent column, to a sump. The sludge normally then is pumped from the sump. While many of these devices have been successfully operated, a problem has existed that at low sludge flows inclusions of air can be sucked into the column sludge inlet ports and into the down pipe which can cause cavitation in the pump or uneven flow into the sump. Also, the open top nature of the sludge sight well allows wind-blown debris or items dropped from the clarifier bridge to foul or obstruct the conduit to the sump or pump suction.
Another prior art construction is termed a Duct-Manifold type collector in which a perforated duct is in communication with a rotating bottomless drum at the center tank bottom, with a sludge withdrawal opening in the concrete tank floor below the drum leading directly into a sludge sump or pump suction. A limitation of this device has been that the duct orifices, which are located at intervals along the arm, and of varying diameters to provide uniform clarifier bottom sludge pickup, are usually of small enough size that settled trash and debris are sucked into the smaller orifices, plugging them and upsetting balanced flow.
Another prior art device known as a Top-Feed type collector as seen in U.S. Pat. No. 4,193,877 includes an open-top collection box forming an influent energy dispersion trough and baffles adjacent to influent windows in an influent column at the liquid/solids level in the tank. The sludge/liquid level in the box is adjustable. Settled sludge is transported upward through draw-off pipes into the box and into an inner concentric pipe in the influent column. Again since the box has an open top, air and trash inclusions can be present in the pumped sludge discharge and pump cavitation or clogging can result.
In both cases of concentric internal sludge discharge pipes, an additional consideration comes into play when the steel parts of the clarifier wear out, and the owner considers replacement with either an identical model, or a newer concept unit. Duplicating the original unit is possible without modifying the tank bottom. However if a Duct Manifold type unit is now wanted, the thick concrete center floor area must be excavated and modified to suit. The present invention makes possible the changeover from an "open top sight well" type clarifier to a "duct manifold" type clarifier, without requiring excavation of existing concrete tank bottom.
Another prior art device offered by several manufacturers is termed the Duct-Sightwell type collector in which a perforated duct near floor level is in communication with an open top sightwell. That sightwell carries the drawback of the open top designs already mentioned.